Micro-mechanical switch and method for making same

ABSTRACT

The micromechanical switch comprises a deformable bridge ( 1 ), attached via its ends to a substrate ( 2 ), and actuating means ( 3 ) to deform the deformable bridge ( 1 ) so as to make an electric contact between a first conducting element ( 4 ) formed on the substrate ( 2 ), between the bridge ( 1 ) and the substrate ( 2 ), and a second conducting element ( 5 ), securedly affixed to a bottom face of the bridge. The second conducting element ( 5 ) is permanently connected, by means of a conducting line ( 6 ) securedly affixed to the bridge ( 1 ), to a third conducting element ( 7 ) arranged on the substrate ( 2 ) at the periphery of the bridge ( 1 ). The bridge ( 1 ) comprises a first insulating layer wherein a hole ( 10 ) is drilled, in which hole a conducting material is arranged salient from the bottom face of the bridge ( 1 ) so as to form the second conducting element ( 5 ).

BACKGROUND OF THE INVENTION

The invention relates to a micromechanical switch comprising adeformable bridge, attached via its ends to a substrate, and actuatingmeans to deform the deformable bridge so as to make an electricalcontact between a first conducting element securedly affixed to thesubstrate and arranged between the bridge and the substrate, and a thirdconducting element arranged on the substrate at the periphery of thebridge.

STATE OF THE ART

Micromechanical switches often present problems concerning the contactresistances. For example, the contact resistance may fluctuate in timeor be too high when the contact is not sufficiently intimate.

To switch a radiofrequency signal with a micromechanical switch, a knownembodiment comprises a deformable bridge and first conducting elementsdesigned to be connected to one another, arranged on a substrate betweenthe substrate and the bridge. The bridge comprises a second conductingelement on the bottom face thereof. The electrical contact between thefirst conducting elements is made when the bridge is deformed byactuating means so that the second conducting element touches all thefirst conducting elements. This however constitutes a hyperstaticstructure (comparable with a table with four legs where one leg issuperfluous), i.e. only one of the contacts is intimate and presents alow contact resistance whereas the contact resistances of the othercontacts are higher. To ensure that the contact resistances of thedifferent electrical contacts are substantially equal, a very greatprecision would be required when manufacturing the switch, which wouldmake production thereof difficult and costly.

The document WO02/01584 describes a micromechanical switch comprising ametal bridge arranged on a substrate and deformable by means of anelectrostatic actuator, and a conducting element arranged between thebridge and the substrate. Actuation of the electrostatic actuator causesdeformation of the bridge so as to make an electrical contact betweenthe bridge and the conducting element. The bridge can undergo strainhardening with use, which may lead to breaking thereof.

OBJECT OF THE INVENTION

The object of the invention is to remedy these shortcomings and moreparticularly to achieve a more robust switch, while avoiding hyperstaticstructure problems.

According to the invention, this object is achieved by the appendedclaims and in particular by the fact that the deformable bridgecomprises at least a first insulating layer wherein a hole is drilled,in which hole a conducting material is arranged salient from the bottomface of the bridge so as to form a second conducting element designed tocome into contact with the first conducting element when deformation ofthe bridge takes place, a conducting line connecting the secondconducting element to the third conducting element being arranged on thefirst insulating layer.

The invention also relates to a process for production of a switchaccording to the invention, wherein fabrication of the deformable bridgeis achieved by:

-   -   deposition of a sacrificial layer above the first conducting        element,    -   deposition of a first insulating layer on the sacrificial layer,    -   etching of a hole in the first insulating layer and in the        sacrificial layer,    -   deposition of a metal layer so as to fill the hole and form the        second conducting element and the conducting line,    -   removal of the sacrificial layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIG. 1 represents a micromechanical switch according to the prior art.

FIG. 2 represents a micromechanical switch according to the invention.

FIG. 3 represents a preferred embodiment of a micromechanical switchaccording to the invention.

FIG. 4 represents a top view of an embodiment of a switch according tothe invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

The micromechanical switch represented in FIG. 1 is composed of adeformable bridge 1 attached via its ends to a substrate 2, andactuating means 3 a and 3 b designed to deform the deformable bridge 1so as to make an electrical contact between first conducting elements 4(three in FIG. 1) formed on the substrate 2 between the bridge 1 andsubstrate 2, and a second conducting element 5 securedly affixed to abottom face of the bridge 1. This switch according to the prior artmakes electrical contact between the first conducting elements 4 whenthe actuating means 3 deform the bridge 1.

In the micromechanical switch represented in FIG. 2, the secondconducting element 5 is permanently connected by means of a conductingline 6 securedly affixed to the bridge 1 to a third conducting element 7arranged on the substrate 2 at the periphery of the bridge 1.Deformation of the bridge 1 makes an electrical contact, by means of theconducting line 6 and the second conducting element 5, between the thirdconducting element 7 and a single first conducting element 4, arrangedfacing the second conducting element 5.

In FIG. 2, the deformable bridge 1 is formed by a first insulating layerwherein a hole 10 is drilled, in which hole a conducting material isarranged salient from the bottom face of the bridge 1 so as to form asecond conducting element 5 designed to come into contact with the firstconducting element when deformation of the bridge 1 takes place. Thus,the bottom face of the bridge 1 is made of insulating material. Aconducting line 6, arranged on the first insulating layer, connects thesecond conducting element 5 to the third conducting element 7.

The deformable bridge 1 can be formed by superposition of thin layers.Thus, a conducting layer constituting the conducting line 6 andconnecting the second conducting element 5 and the third conductingelement 7 can be formed on the first insulating layer. In an alternativeembodiment, the second conducting element 5 and the conducting line 6can be formed by a single conducting layer.

As represented in FIG. 3, a second insulating layer 8 can be formedabove the conducting line 6.

In the switch represented in FIG. 3, a conducting line 6 connects thesecond conducting element 5 to two third conducting elements 7 arrangedon each side of the bridge 1. The bridge 1 can comprise an insulatinglayer 8 above the conducting line 6. An insulating layer 9 is preferablyarranged between the first conducting element 4 and the substrate 2, theinsulating layer 9 having smaller lateral dimensions than the lateraldimensions of the first conducting element 4, so that the firstconducting element 4 is convex. Due to the convex shape of the firstconducting element 4, the contact between the first conducting element 4and the second conducting element 5 forms a localized contact at thecenter of the hump.

A switch according to the invention presents the advantage of beingrobust and of having a single contact which can be made sufficientlyintimate by a suitable actuation. The contact resistance is consequentlyvery low.

For example, the micromechanical switch can be a normally openradiofrequency switch, the actuating means 3 comprising an electrostaticactuator. In this case, as represented in FIG. 4, the first conductingelement 4 is a radiofrequency line. When the switch is open, theradiofrequency signal can pass via the radiofrequency line forming thefirst conducting element 4, contact losses thus being prevented. Theactuating means 3 are preferably formed by electrodes 3 a and 3 b of anelectrostatic actuator. The electrodes 3 a can be arranged in the firstinsulating layer of the bridge 1, as represented in FIG. 3. Theelectrodes 3 a, securedly affixed to the bridge 1, are connected to avoltage source. The electrodes 3 b, formed on the substrate 2, betweenthe deformable bridge 1 and the substrate 2, on each side of theradiofrequency line constituting the first conducting element 4, formtwo ground planes substantially parallel to the radiofrequency line.They thus perform a twofold function. Firstly, the electrodes 3 b enablean attractive electric force to be established between the electrodes 3a and the electrodes 3 b enabling the bridge 1 to be deformed when avoltage is applied between the electrodes 3 a and 3 b. Secondly, theelectrodes 3 b act as wave guide for the signal transmitted by theradiofrequency line constituting the first conducting element 4. In theapplication considered, the third conducting elements 7 are formed byelectric ground planes arranged on the substrate 2 on each side of thedeformable bridge 1. Thus, actuation of the switch establishes a contactbetween the radiofrequency line and the electric ground planesconstituting the third conducting elements 7. The electric signal isthen absorbed by the electric ground. The radiofrequency switchdescribed above presents the advantage, in the on state, of transmittingthe radiofrequency signal without any contact loss.

The whole of the radiofrequency component can be achieved on thesubstrate 2 by conventional integrated circuit fabrication techniques.The surface of the substrate 2, whereon the third and first conductingelements 4 and 7 are arranged, has to be made of insulating material toprevent permanent short-circuiting of the conducting elements. Theinsulating material is typically silicon oxide. In a preferredembodiment, an insulating layer 9 is deposited on the substrate 2 at thelocations of the electrodes 3 b and at the location of the firstconducting element 4, the insulating layer 9 having smaller lateraldimensions than the lateral dimensions of the electrodes 3 b and of thefirst conducting element 4 respectively. The material of the insulatinglayer 9 can for example be Si3N4 or SiO2. The first conducting element 4and the electrodes 3 b can be deposited on the insulating layer 9 bydeposition of a metal layer, preferably of gold. The sacrificial layercan then be deposited above the first conducting element 4 and theelectrodes 3 b. The material of the sacrificial layer is typically apolymer material able to be easily removed after fabrication of thebridge. On the sacrificial layer, a layer of insulating material formingthe framework of the bridge 1 is deposited. The insulating material ofthis layer can for example be Si3N4 or SiO2. To achieve an electrostaticactuator, the electrodes 3 a can be fabricated by a metal deposition onthe insulating layer forming the framework of the bridge 1 and coveringof the electrodes 3 a by an additional insulating layer (not shown)designed to insulate the electrodes 3 a from the conducting line 6. Thehole 10 is drilled by etching in the insulating layer forming theframework of the bridge 1, in the additional insulating layer and in thesacrificial layer. The second conducting element 5 and the conductingline 6 are then achieved, preferably simultaneously, by depositing ametal layer so as to fill the hole 10 and form a layer connecting thesecond conducting element 5 and the third conducting element 7.Preferably, a second insulating layer 8 (Si3N4 or SiO2) is depositedabove the conducting elements. The sacrificial layer is then removed.

1-10. (canceled)
 11. Micromechanical switch, comprising a deformablebridge, attached via its ends to a substrate, and actuating means todeform the deformable bridge so as to make an electrical contact betweena first conducting element securedly affixed to the substrate andarranged between the bridge and the substrate, and a third conductingelement arranged on the substrate at the periphery of the bridge, switchwherein the deformable bridge comprises at least a first insulatinglayer wherein a hole is drilled, in which hole a conducting material isarranged salient from the bottom face of the bridge so as to form asecond conducting element designed to come into contact with the firstconducting element when deformation of the bridge takes place, aconducting line connecting the second conducting element to the thirdconducting element being arranged on the first insulating layer. 12.Switch according to claim 11, wherein the actuating means comprise anelectrostatic actuator.
 13. Switch according to claim 12, wherein theelectrostatic actuator comprises electrodes arranged in the firstinsulating layer of the bridge.
 14. Switch according to claim 11,wherein the first conducting element is a radiofrequency line and thethird conducting element is an electric ground plane arranged on thesubstrate.
 15. Switch according to claim 11, wherein two ground planesare arranged on the substrate on each side of the bridge and connectedto the second conducting element, the conducting line connecting thesecond conducting element to the two ground planes.
 16. Switch accordingto claim 11, wherein the deformable bridge comprises at least oneconducting layer forming the conducting line.
 17. Switch according toclaim 16, wherein the second conducting element and the conducting lineare formed by a single conducting layer.
 18. Switch according to claim11, wherein the deformable bridge comprises at least a second insulatinglayer above the conducting line.
 19. Switch according to claim 11,wherein a third insulating layer is arranged between the firstconducting element and the substrate, the third insulating layer havingsmaller lateral dimensions than the lateral dimensions of the firstconducting element, so that the first conducting element is convex. 20.Process for production of a micromechanical switch according to claim11, comprising fabrication of the deformable bridge by: deposition of asacrificial layer above the first conducting element, deposition of afirst insulating layer on the sacrificial layer, etching of a hole inthe first insulating layer and in the sacrificial layer, deposition of ametal layer so as to fill the hole and form the second conductingelement and the conducting line, removal of the sacrificial layer.